text:
When must we make contact with ATC?
If I, pilot, have to make a stop at the undesignated parking position for some reason, such as FOD or ground handlers crossing in the ramp area, it is mandated that I have to contact ATC to explain why I have to stop?
Is there any global reference that describing when flight crews have to make a contact with ATC?

FOD is something that ATC would want to know about, so they can clear it. Ground handlers crossing your cleared path is an incursion, and would also want to know about it. I'm not sure what you are asking? Any time you deviate from an ATC instruction you need to talk to them.

Not really talking to ATC at that point, you'd be talking to Gnd Control.

I'm not sure what you're asking. Does "undesignated parking position" mean somewhere in the movement area or in the non-movement area? For your second question about when to contact ATC in general, this question might be helpful. But the requirements to contact ATC will be different in different countries and for different types of flight (e.g. VFR vs IFR). If you can give us more details about your scenario, that would be helpful.

Is there any global reference describing when flight crews have to make contact with ATC?

On the ground, you need a clearance from ATC when you are on the maneuvering area, which consists of taxiways and runways. The ramp/apron is typically not part of the maneuvering area, and as such not controlled by ATC, although many large airports have some sort of ground control for these area.
In the air, you need a clearance from ATC whenever you are in controlled airspace. Some countries additionally require pilots flying IFR to be in contact with ATC even in uncontrolled airspace.
Note: ‘controlled airspace’ means an airspace of defined dimensions within which air traffic control service is provided in accordance with the airspace classification; (from SERA, EU 923/2012)

The thing is that you need clearance to move. If you stop, and for unexpected reason on top of that…

"In the air, you need a clearance from ATC whenever you are in controlled airspace." - Class E airspace is controlled airspace, but you don't need a clearance to fly VFR there. Maybe you can clarify this point?

@TannerSwett Class E is controlled airspace for IFR flights, but not for VFR

The FAA's Pilot Handbook of Aeronautical Knowledge simply says that Class E is controlled airspace; it doesn't say anything like "controlled for IFR and uncontrolled for VFR". Maybe there are other parts of the world where Class E airspace is considered uncontrolled for VFR traffic?

@TannerSwett I can't really adjust my answer to take into account the deviant rules of one single country

I looked up what information I could find about the UK and Japan and found that in both of those countries, Class E is simply described as controlled airspace, not as "controlled for IFR and uncontrolled for VFR". So may I ask for an example of a country where Class E is not controlled airspace for VFR flights?

@TannerSwett I've added the definition for controlled airspace (according to the EU) to my answer. Here is an extract (also from SERA) regarding class E: "Class E. IFR and VFR flights are permitted. IFR flights are provided with air traffic control service and are separated from other IFR flights. All flights receive traffic information, as far as is practical"

Class E is controlled, meaning that ATC service is available. VFR is not required to get a clearance in class E, though, or even talk to ATC at all. This leads many VFR pilots to mistakenly call it uncontrolled.

Whenever you are operating under a clearance—whether on the ground or in the air—you have to inform ATC of any deviations. However, other concerns—controlling the aircraft—take priority. So you actually inform ATC as soon as you can safely do it, after the fact.
When you are cleared to take-off, you are implicitly cleared to abort, when you are cleared to land, you are implicitly cleared to go around and in any other circumstance you have enough free space around you that you won't run into anybody else (properly controlled) if you take some time.
So when you are taxiing along and your engine sucks something up and quits, you first slam on the brakes—because you definitely don't want to hit anything—then you do the memory items from the engine fire or failure checklist and only then you key the mic and announce something like “Tower, N123AB stopped on taxiway K due to engine failure. We might have ingested some foreign object.” (and if the fire alarm came on, ask for the firefighters). The next aircraft behind you either sees you well enough to stop in time, or (if it's really bad fog) has a lot of distance.
The worst case is probably emergency descent, where when you retard to idle and deploy the spoilers, you'll be through the next assigned level 1,000 ft below in about 10 seconds, but that only happens up where everybody is required to have a transponder, so you can probably trust the T-CAS for a bit.
For a real anecdote, I'd quote How It Flies:

I didn’t like what I was imagining, so I pulled the throttle to idle and stomped on the brakes. I also keyed the transmitter and said “Tower, Two-Four-Kilo is gonna hold our position for a moment”.

Read? Stomped on the brakes without having any confirmation that he should—because he had an—absolutely correct—hunch that the other guy would bust his clearance and land on the runway he was cleared to take off from and not the parallel one he's been assigned. You first control the aircraft as safely as you can, and then tell ATC what is going on.


text:
What is the sink rate of a glider during a roll?
For a glider in a steady bank at angle $\mu$, to a good approximation, the sink rate has a factor $1/\sqrt{\cos \mu}$ relative to the sink rate at zero bank angle.
What are the dynamics of the sink rate during a roll, for instance, when rolling from a bank angle of zero to a bank angle of $\mu \sim 20^{\circ}$ in one second?
I'm not looking for a very precise, quantitative answer but rather some qualitative idea of the physical effects that underlie the dynamics of the sink at such roll rates for a model plane.
Any references or guidance will be helpful, thanks!

Would you be able to take the average of the 2 angles (start and finish)? so, the average of 0 and 20 degrees? That would be 10 degrees. so your sink rate would be using that same formula but instead of mu being 20, it would be 10.

@alex the bank rate influence is non-linear, as seen from the 1/sqrt(cos) equation, so applying it to the average of the input (the bank angle) does not provide the average of the output

@Federico Clearly, it is not, but -intuitively- I believe that the form of the curve lift/bank angle is probably concave during the first half of the roll and convex during the second. Thus, the non-linearity may (roughly) cancel out... and a linear rate might be OK for a first approximation...

@xxavier "I believe that the form of the curve lift/bank angle is probably concave during the first half of the roll and convex during the second" it is not

@Federico The lift will drop from max to zero during the first 180º of the roll, and rise from zero to max during the second 180º. It's reasonable to assume that the graph will be symmetrical. Hence, the non-linearity would (more or less) cancel out...

@xxavier but we are speaking about a change from 0 to 20, aren't we?

@Federico Well, sorry, no... I was thinking about the change along the full 360º roll... As I understood that the 20º/second was the rate of roll... Now I see that the OP means the full interval is of 20º...

@xxavier The lift will not necessarily be zero when inverted. It may be negative, or something else depending on angle of attack.

@DJClayworth. Yes, you're right. I should have written that, in a 360º roll, upwards-directed lift may approach zero at 90º and again at 270º... Having a bad day today...

The title is confusing, as is some of the phrasing in the body of the question.  I would suggest replacing "during a roll" with "while rolling (changing bank angle)".  It appears that this change would not invalidate any existing answer.  The phrase "a roll" implies a full 360-degree roll, and those maneuvers are usually initiated in a climb, even in gliders, hence the confusion.  Or, if you really are asking about a full 360-degree roll, please state that explicitly.  Actually-- it may be better to ask about how the drag coefficient increases than how the sink rate increases.

As always, it depends. Besides the steady-state drag sources of lift creation and friction, you need to look at the drag created by roll acceleration and manoeuvring, including adverse yaw.
If the glider starts the roll at a bank angle which is sufficient to reach a steady roll rate by the time it rolls through 0°, the roll acceleration term becomes zero. However, now you have deflected ailerons and a deflected rudder to counter adverse yaw, which all add their own drag. How much depends on the roll rate and the needed deflection.
On top, now the local incidence of the wing shows a discontinuity at the spanwise station where the ailerons begin. This will create a less than optimum lift distribution over span, so the induced drag of the rolling glider is higher than that of the same glider in a steady glide. Again, the magnitude depends on the roll rate, and the aileron placement and deflection angle.
I would not be surprised if the sink rate in a brisk roll increases by 50% over that in a steady glide at the same speed, even with zero sideslip. If adverse yaw is not properly countered by the rudder, the additional drag from the resulting sideslip angle can easily double the sink rate.

Thanks! We have a good amount of data using a model plane. The change in sink rate is about 50% for a turn from $20^{\circ}$ to $40^{\circ}$ as you guessed but the sink rate changes in both directions - it increases when we move the bank angle farther from 0 and decreases when we move it towards 0. The sign suggests this is due to unbalanced lift. This change is not affected when we have a rudder mix in our rolls so it is not due to sideslip nor is it affected when we soften our aileron deflection. We can test if it's due to roll acceleration; a reference for this will be greatly appreciated.

@un_anonymous: Sorry - I do not know of a reference how roll acceleration will increase drag. But this should be straightforward to calculate once you know the roll inertia of the plane and the drag increase of a roll command.

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text:
What exactly distinguishes a beacon light from a strobe light?
Different sources seem to have different opinions:
Wikipedia: Aircraft warning lights:

These lights include landing lights, red or white flashing beacons, wingtip strobes, and wingtip navigation lights.

Beacons and strobes are flashing lights.
Beacons may be red or white.
Strobes are wingtip lights. I interpret this as strobes being directional while beacons cover 180°.

Aeronautical Information Manual (AIM), 4−3−23. Use of Aircraft Lights:

b. An aircraft anti−collision light system can use
  one or more rotating beacons and/or strobe lights, be
  colored either red or white, and have different (higher
  than minimum) intensities when compared to other
  aircraft. Many aircraft have both a rotating beacon
  and a strobe light system.
d. Prop and jet blast forces generated by large
  aircraft have overturned or damaged several smaller
  aircraft taxiing behind them. To avoid similar results,
  and in the interest of preventing upsets and injuries to
  ground personnel from such forces, the FAA
  recommends that air carriers and commercial
  operators turn on their rotating beacons anytime their
  aircraft engines are in operation.

Beacons are rotating lights. They're always referred as rotating beacons in the AIM.
Both beacons and strobes can be either red or white.
I take the sentence Many aircraft have both a rotating beacon
and a strobe light system. as beacons being the lower intensity lights which are constantly turned on during engine operation signalling the danger of an aircraft in operation, while the strobes being the higher intensity flashing ligths with the reason of being seen from great distance.

Wikipedia: Aviation navigation lights

Aircraft navigation lights are placed in a way similar to that of marine vessels, with a red navigation light located on the left wingtip leading edge and a green light on the right wingtip leading edge. A white navigation light is as far aft as possible on the tail or each wing tip.[3] High-intensity strobe lights are located on the aircraft to aid in collision avoidance.[4]
In civil aviation, pilots must keep navigation lights on from sunset to sunrise. High-intensity white strobe lights are part of the anti-collision light system, as well as the red rotating beacon.
All aircraft built after 11 March 1996 must have an anti-collision light system (strobe lights or rotating beacon) turned on for all flight activities in poor visibility. The anti-collision system is recommended in good visibility, where only strobes and beacon are required. For example, just before pushback, the pilot must keep the beacon lights on to notify ground crews that the engines are about to start. These beacon lights stay on for the duration of the flight. While taxiing, the taxi lights are on. When coming onto the runway, the taxi lights go off and the landing lights and strobes go on. When passing 10,000 feet, the landing lights are no longer required, and the pilot can elect to turn them off. The same cycle in reverse order applies when landing.

Beacons are red rotating lights.
Strobes are high-intensity white lights.
Beacons stay on for the duration of the flight including the whole engine run time.
Strobes are added from takeoff until leaving the runway after landing.

Is there a clear definition?

Welcome to Aviation.SE. Really nicely worked out question!

@SentryRaven Thanks!

I think there are two points in your question: what is the physical difference between a beacon and a strobe, and what is the operational difference? For the rest of this answer, I'm going to assume that you're asking about an aircraft that has both a beacon and strobes; as you quoted above, it's possible for an aircraft to have neither. I'm also going to assume you're asking for an answer that applies in the US; regulations in other countries may be different.
First, the physical difference is exactly what you've already determined: a beacon is a (relatively) low-intensity rotating red or red/white light; a strobe is a high-intensity flashing white light. (As noted in the comments below, some beacons flash instead of rotating, but they're still lower intensity than a strobe.) This isn't formally defined anywhere (e.g. 14 CFR 1.1 doesn't define them) but the information you found in the AIM is exactly how everyone I know uses the terms. It's also how you find those words used on aircraft part sites, e.g. beacons and strobes. So whatever the 'strict' definition might be, the practical and accepted one is exactly what you've found out.
Second, the operational difference essentially comes down to the question "when do I have to turn them on?" and in addition to what you found in the AIM the FAA gave more details as part of this legal interpretation. The questioner asked two questions about beacons and strobes: when to turn them on, and if they both have to be turned on. The key regulation quoted is 14 CFR 91.209, and note that the FAA considers the beacon and the strobes together to form a single "anticollision system":

[...] it appears that the strobe light and the rotating beacon are
part of the same anticollision system.

When to turn them on:

91.209(b) requires that an aircraft's anticollision lights be turned
on once that aircraft's engine is started for the purpose of air
navigation. As a safety precaution, the anticollision lights should be
on before starting an engine or causing a propeller or rotor to move.

If both have to be turned on:

[...] because section 91.209(b) does not contain an exception for
alternative sources of anticollision lighting, turning on the
anticollision beacon would not relieve a pilot from the requirement to
turn on the anticollision strobe lights. See § 91.209(b). However, §
91.209(b), does give the PIC the discretion to turn off the anticollision beacon and/or the anticollision strobe light system if
the PIC determines that it is in the interests of safety to turn off
either one or both of these components of the same lighting system.

In other words, the regulations say you should turn them both on, but as PIC it's up to you to determine if using them is safe or not, and if not then you are allowed to turn (or leave) them off.
Practically speaking, I've always been taught to turn the beacon on before starting the engine because it's there to tell people near the aircraft that the engine is running or about to be running. The strobes should go on any time you need to be highly visible (which is most of the time), but not if using them would blind someone else or even yourself, e.g. taxiing or flying in clouds. The AIM 4-3-23 has more details on that, in addition to the parts that you quoted.

Isn't it SOP to turn strobes on immediately before entering the runway and off on vacating it?  Strobes on during ground operations is not a good thing.

@Simon Yes, that's a standard practice. But the AIM recommends turning on all lights when taxiing across a runway, for example. The point of the FAA's commentary is that they want to be flexible and allow the pilot to determine when it's safe and appropriate to use the strobes (or other lights).

It's worth noting that not all beacons are of the "rotating" variety. Many Cessnas use "flashers" (halogen lamps that flash periodically, but aren't the rapid-discharge high-intensity type of light you get from a strobe) as their beacon lamp.

Pondlife, this is a great answer, thank you. And thanks to @voretaq7 for confirming that there are non-rotating beacons.

@voretaq7 Thanks for that comment, I added it into my answer

As part of the pre-cloud penetration procedure I used to turn the strobes, landing light and rotating beacon off to prevent glare from the light bouncing back off the clouds and destroying night vision, and to prevent the disorienting illusion of turning left created by the tail-mounted rotating beacon rotating to the right. Also, I don't believe the landing light is required at all. I've flown at night without one (inop).

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text:
Are there any aircraft that come with interchangeable flightdecks?
With the new Boeing 787 where Boeing has provided the capability to swap engine types if the aircraft goes to a new operator quite quickly, I'm wondering if there are any interchangeable flightdecks?
Say that you might have a legacy 737 for southwest, but an entirely different cockpit layout (containing the same capabilities) for a company which operates Boeing 787s as well, since the similarities would make training easier.
I know a similar project was done on the DC-10s becoming MD-10s, as well as some Saudi MD-90s to be similar to MD-11, but both of these were long-time consuming projects.
But I'm wondering if there are any aircraft which have this possibility and if not, why not? I'd see it as an opportunity to Boeing to have a 787 flightdeck shared with say the 777.

According to this answer they used to be very customizable, but not very much interchangeable at least to the point that no one bothered to change them on second-hand aircraft. They are reducing it because it only causes confusion. Aircraft change owners and pilots change employers and it's most useful if all aircraft of the same type have the same cockpit layout.
As far as 787 goes, it has very similar flight deck to 777, always, to the point it has common type rating.

With EFIS glass cockpits data specific to an aircraft type from the Flight Computer are displayed on the screen.
That screen and a standard layout however could be a common feature on several aircraft from the same manufacturer which as Jan Hudec points out lead to common type ratings allowing pilots to be cross rated on more than one aircraft.


text:
Does Qantas have an above-average safety record?
There is a widespread belief, at least within Australia, that Qantas has, or had, an above-average safety record. For example, AirlineRatings.com recently listed Qantas as the world's safest airline, which was reported on by the Australian Broadcasting Corporation and Fairfax Media. Personally speaking, it seems like aircraft incidents with Qantas or JetStar get more coverage than those involving other airlines, but I can't verify that.
I'm skeptical of the belief that Qantas has an above-average safety record, let alone being the safest airline in the world.
From the Wikipedia article List of Qantas fatal accidents, I know that Qantas hasn't had any fatal jet airliner incidents, and the last accident it had was in 1951. However, I don't know whether aircraft accidents are so rare that an airline can have a better average than others because of chance alone.
I'm not doubting that some countries have safer airlines than others. If offered the choice between Qantas and Air Koryo, I'd definitely choose the former. I also don't doubt that some countries are safer to fly in than others - for example, planes in Australia don't have to deal with snow very often. However, I don't expect that to be a major factor. So I'm ultimately interested in a comparison between Qantas and airlines from countries that are comparable to Australia.
Did Qantas have a perception of having an extraordinary safety record before the 1988 movie Rain Man, where the titular character said that Qantas never crashed?
Is there enough data to say that Qantas is the safest airline in the world, or even say that it's safer than similar airlines, such as the ones Raymond refused to get on in the movie?

This question asks deliberately for opinions (e.g. Is there enough data to say that Qantas is the safest airline in the world), without listing unquestionable criteria. In addition the way it's written makes the question looking like a statement on Quanta's quality more than a way to satisfy curiosity or a practical need.

@mins Perhaps just sceptical and does not want to trust the 'opinion' presented to him. Not everybody knows or understands such criteria either, leading to an opinion in some capacity being necessary from somebody who may know. I agree it's not a perfect objective question but I think it's a legitimate query.

Accidents are so rare, that the underlying statistics are not useful.  Imagine 20 airlines with no accidents.  Now one has an accident, not it's fault, let's say multiple bird strikes.  Now it's safety record is infinitely worse than the others.  I think the most accurate statement you can make is that all of the major western airlines have an equally good safety record.

@Simon: True. I'd add that the "safest", in spite of appearances, is the one that anticipates problems and try to prevent them by maintenance, training, and keeping crews dedicated. This is not something advertised. The safest is not the one that never had accidents. No accident in the past is a measure of the past, prevention is telling for the future.

Defining and Measuring 'Safety'
These ranking depends how you define safety. Here is your problem: You can easily do this by widely publicised facts, such as well documented fatal accidents. Fatal accidents occur much much too seldom to with any statistical confidence to say how safe an airline is.
What you would much rather do is go by accidents and incidents (the little things that occur everyday and give you a feeling for how stuff is done and handled). For instance: hydraulic leaks, loss of separation to other aircraft or electrical problems prompting a return. A small selection of these are reported on the popular site Aviation Herald, but this is (emphasis) only a small subset that happen to come through. Some western countries with the objectively the best safety are over-reported since they are just better at documenting. You'll find heaps of entries on 'fume events' on Airbus aircraft from Germany for instance.
These are considerably harder to collect, since how they are classified and reported changes by country and aviation agency. Some publish for common good, other ones barely collect them. For this reason, the way these 'rankings' do it is by fatal accidents and other easy to get parameters. This is eventually what comes through to newspaper sites with little detail on how they were put together.
The parameters measured by AirlineReports:

Is the airline IOSA certified?
Is the airline on the European Union (EU) Blacklist?
Has the airline maintained a fatality free record for the past 10 years?
Is the airline FAA endorsed?
Does the country of airline origin meet all 8 ICAO safety parameters?
Has the airline's fleet been grounded by the country's governing aviation safety authority due to safety concerns?
Does the airline operate only Russian built aircraft?

Most of these are bare minimums that can be expected from an airline, but do not really go to any distance to show who stands out. For any more detailed sense and purposes (how media are using it), I'd call this useless.
Also note that this does not include the liability question for fatal accidents- who was at fault? You can finally also (albeit unlikely) have a completely safe airline that does not have a single certification.
Qantas
Qantas has had two very serious incidents with a Boeing 747 and A380. They also had a serious incident on involving A330 losing altitude. This does not really IMHO award them as the 'safest airline' for the past few years, since this is considerably more than a lot of other airlines have experienced.
However, for all practical sense and purposes, they are as safe as any other airline. The chances of anything happening are just so incredibly remote that it's not worth the trouble of thinking about it. Getting a connecting flight to fly with another airline will probably be less safe than just flying them directly.
As for the maintenance matters, Qantas owns JetStar and they appear to use the same maintenance facility for their aircraft. A further difference is the training where there appear to be overlaps as well, so there are only small differences on perhaps the most important parameters differing the two.

Note that the main criteria for inclusion in EU blacklist seems to be lack of governmental oversight. EASA does not really have enough information to judge individual airlines operating outside its region of authority. All they can is review materials published by the overseeing authority. So the EU blacklist in most cases says “all airlines in country X”, sometimes with exception of some large international ones that have enough interaction with other, more trustworthy authorities.


text:
Does Air Force One (VC-25) have retractable staircases on all its exit doors?
While going through an article on AFO, I came across this little piece of info:

So, does it have retractable steps on all its exit doors? How does it manage doors like these?

I would say that your second photo shows pretty clearly that the statement contained in the first one is false.

@frederico The second photo shows they don't always use a retractible staircase, not that it doesn't have one nor that it isn't used in less-secure environments.

@kevin (it's FEderico, no "r") no, you can see that there is no staircase: it is neither behind the door nor there is a staircase van beneath the door itself.

If it's retractible, why would they extend it if they have a real staircase? Also, that's a different door, I wouldn't necessarily assume it's both or neither. It certainly makes sense to have a retractible staircase in case they should ever have to land somewhere without one.

If they have a retractable staircase, then is it that they have no emergency chutes (or whatever they are called)

A "regular" commercial 747 would have an escape hatch at the top (just above and behind the cockpit windows) with inertial reels that can be used as a "last resort" evacuation.  As the AF1 models have design changes (such as in-flight refueling), I'm not sure if that hatch remains.  http://cdn-www.airliners.net/aviation-photos/middle/7/6/7/0968767.jpg

@Kevin The claim is that each exit has a staircase; the existence of an exit without a staircase means the claim is false. There is no staircase on that exit. Therefore, the claim is false. In fact, they have a staircase on some doors; this is clearly shown by image one. They also use ground-provided staircases when possible, as shown in image two.

@Firee Since they don't have staircases on all doors, they probably have plenty of evacuation slides (it being a military aircraft, I don't think it needs to meet FAA requirements; however, I can't imagine them not putting slides on every door that has no staircase).

@Firee An exit door would need slides to deploy even if that door is equipped with airstairs because in the event of an emergency evacuation, the airstairs may be unusable for any number of reasons.  No aircraft power, damage to the airstairs mechanism, or the aircraft in a position where the airstairs couldn't deploy properly would all make a slide necessary in case of an emergency evacuation.

There is page by cryptome with documentation about AF1 rescue options.
According to the "TECHNICAL ORDER 00-105E-9, 1 FEBRUARY 2006, REVISION 11." (15 April 2006), slide 71 "AIRSTAIR OPERATION VC-25A", Boeing VC-25 has only two exits equipped with own airstairs, only at doors from lower compartments. Doors at main deck have no integrated airstairs.

Source
First photo in the question shows forward airstairs operation, and there is photo of aft airstairs, used in 2006.
According to this slide from the same PDF, there are 5 pairs of entry doors on main deck by design of the Boeing 747-200 plane; but 3 or them (1R, 3L, 3R) are blocked on VC-25 and can't be used. All doors from main deck have no own retractable stairs.
Also, from here:

Unlike a normal 747, the plane has its own retractable stairways, for the rear entrance and the front entrance. These stairways open onto the lower deck, and crew members and staff climb internal staircases to get to the upper decks.

Both stairs used: https://commons.wikimedia.org/wiki/File:The_two_Boeing_VC-25A_Air_Force_One.jpg ("File:The two Boeing VC-25A Air Force One.jpg" 23 October 2003, Hawaii, by CPL Roman Gray, USMC)

It doesn't have retractable stairs on every door; the second picture shows mobile stairs to the upper exit (you can see the driver's cabin behind the crowd).
If mobile stairs aren't available the president will descend to the lower level and use the built in stairs at the lower exit as shown in the first picture. If they are available (and deemed to be safe by the bodyguards) they will be used.

Would I be correct in assuming that this is at least 90% of the reason the lower exit exists in the first place?

You can't put a number on it like that, but it's the primary reason that there is a lower exit, yes.

They're clearly using a mobile truck in that picture, but are you sure there aren't retractable airstairs in the floor by that door?

@cpast, The 747 was notably designed with both passenger and cargo operation in mind. The lower deck doors on the VC-25 are in the same position (though reduced in size I think) as the lower cargo doors on a 747 cargo variant. I imagine the original airframe design for the 747 always had structural provision for openings in those positions.

@RedGrittyBrick, the lower forward airstair on the VC-25 are unique to the two aircraft and were custom designed for that aircraft. It also required major redesign of the fuel and hydraulic system. The initial intent was in the event the aircraft went to an airport that didn't have a 747 ladder truck. I was on the USAF team that bought the aircraft and was in most of the design meetings for that feature. Don't know when the aft airstair was added.

@RedGrittyBrick the lower cargo doors are on the other side.


text:
Does laser eye surgery prevent me from getting an EASA pilot licence?
I am a masters student in Germany and I am very much interested in pilot training. I had a laser eye surgery (LASIK) 3 years ago. I heard that people who got it are not eligible as a pilot. Is this true for a private licence too?

Updated the question to include "EASA" since it is the framework Germany and a bunch of other countries use.

Welcome to Aviation.se

LASIK is a type of refractive eye surgery and according to EASA's medical standards, you can be considered "fit" to fly after surgery if several conditions are met. The conditions depend on the class of medical; private pilots usually have a class 2 medical.
For class 1:

(1)   After refractive surgery,  a fit assessment may be considered,
  provided that:
  (i)    pre-operative refraction was not greater than
  +5 dioptres;
  (ii)  post-operative stability of refraction has been achieved (less than 0.75  dioptres variation diurnally);
  (iii) examination of the eye shows no post-operative complications;
  (iv)  glare sensitivity is within normal standards;
  (v)   mesopic contrast
  sensitivity is not impaired;
  (vi)   review is undertaken by an eye
  specialist.

For class 2 (private pilot) and LAPL:

After refractive surgery a  fit assessment may be considered provided
  that there is  stability of refraction, there are no postoperative
  complications and no increase in  glare sensitivity.

The LBA also has an FAQ on vision issues but it doesn't mention LASIK. It does say, however, that you need an optician to examine you if there are any questions about your vision that an aviation examiner can't determine himself (nicht durch den flugmedizinischen Sachverständigen durchgeführt werden können). I assume that means that if there's any doubt about your eyes, you'll need to have an additional eye exam.

In the general case, no, it is not universally disqualifying. That said, general advice doesn't substitute for talking to a flight surgeon who can issue you a medical certificate where you are, and determining exactly what will be required you you, given your history, your procedure, and the rules there in Germany.
But since there are pilots now who've had LASIK in the past, it's definitely worth scheduling the appointment & having that conversation. In the past, when LASIK was newer & less proven, the rules were much more restrictive, but they are much less so today.  Good luck!
Edit: this link is specific to the FAA, but it gives a flavor at least for attitudes in the US medical community toward LASIK. Hopefully European standards are similar.


text:
Is it legal to use RC quadcopters for commercial purposes?

Is it legal to fly a drone with a go pro or something and take photos and video for commercial purposes?
This is what I found on the FAA website.

All UAS operations for commercial or business purposes are subject to FAA regulation. At a minimum, any such flights require a certified aircraft and a certificated pilot. UAS operations for commercial or business purposes cannot be operated under the special rule for model aircraft found in section 336 of Public Law 112-95.

http://www.faa.gov/about/initiatives/uas/uas_faq/
Many news articles say that the FAA will not really care.

For non-commercial uses, a rc-controlled model aircraft, even a quad-copter, the FAA doesn't, yet. In fact, there are provisions which say that they cannot regulate them. For commercial usage, however, they do care. As you yourself have read, commercial usage precludes them being considered as model (hobbiest) craft. There is at least one on-going court case over FAA regulation or not. This being a legal question, I will not submit an answer, as I've already been bitten once. I would strongly suggest you (your lawyer actually) research fully.

Is it legal to fly a drone with a go pro or something and take photos and video for commercial purposes?

Yes, many news businesses think so

the numbers of drones licensed for commercial flights by the nation's Civil Aviation Authority (CAA), which regulates the use of unmanned flying aircraft, shoots up -- from 30 in January 2013 to more than 300 today

Many news articles say that the FAA will not really care.

Most drone operators will not care too much what the FAA thinks. This is because the FAA don't have authority over most drone operations. This is mainly because they only really have authority in one country in one part of one continent. The FAA won't be arresting Mr 王 in 中国.
You can read what the FAA says about Unmanned aircraft.

When the FAA discovers apparent unauthorized UAS operations, the agency has a number of enforcement tools available to address these operations, including a verbal warning, a warning letter, and an order to stop the operation.

Which suggests the FAA cares, at least about commercial UAS operations in the northern parts of the continent of America which aren't Canada or Mexico.
They also have a FAQ

All UAS operations for commercial or business purposes are subject to FAA regulation. At a minimum, any such flights require a certified aircraft and a certificated pilot. UAS operations for commercial or business purposes cannot be operated under the special rule for model aircraft found in section 336 of Public Law 112-95.

I'm not sure your answer is formatted in a way that is ... I don't know if ethical is too strong ... The OP references FAA, which indicates s/he is interested in U.S. regs. Yet in your answer you start out with a YES answer, but the link is to the UK. It seems the answer is a qualified YES, not an unqualified one. The qualifications are set forth by the FAA.

@CGCampbell: The first link in the answer is to an article which is primarily from a UK perspective but does discuss the FAA and the situation in the USA. At the time I wrote the answer, although the body of the question mentions FAA, neither the question's title nor its tags mentioned a country or authority. So I suspect searchers with wider interests are likely to find this answer.

Here's the thing. Unmanned Aerial Systems, whether you call them drones, or hobby craft, are (IMHO) going to be subject to ever tightening controls/regulations. With the competing requirements of governments and the desires of the public at large over privacy, crime control, prevention, and airspace saturation (below x ft is uncontrolled (?) until two drones run by, say, amazon and google, collide and burn down an apartment complex), I don't see answers given to this question today being correct for months, let alone years.

aand for the same reasoning, I can't see giving a broad legal opinion for anything other than one country. If you search rc-modelers forum there is a thread that is VERY devisive over perceptions and intretations of the laws that exist, just in the US.

THE RULES HAVE CHANGED.  This question should be deleted and re-asked.
The generally relevant rules for commercial drone piloting are found in 14 CFR 107.  However, the prudent student will be reminded that there are other FARs which are relevant, including 14 CFR 91, NTSB 830 and for hobbists (non business use) 14 CFR 101 plus the rules of a CBO.

The FAA has an updated page that is a good place to start. https://www.faa.gov/uas and if you follow the links, they also cover getting your license so you can fly commercially. https://www.faa.gov/uas/getting_started/fly_for_work_business/becoming_a_pilot/


text:
What is this object that followed me up the street?
I saw my first UAV last night as it followed me up the street in Lancaster, PA. Does anyone know what type of unit this is or who would own one? I am guessing it was the police? I am curious.

This is a quadcopter, so called because it uses four rotors to generate lift. They are now made cheaply in China and can be purchased in most technology-related online shops. They are remotely controlled, so it is safe to assume that the owner was close, but not in plain view. It might have been the police, but it could as well be a hobbyist, taking his new toy out for a spin.
Since most units are equipped with a cheap webcam and a wireless link, I would guess he/she was sitting in front of a computer or using his/her smartphone for controlling the device and watching the camera stream.

There is one available (not quite the same unit) at my local Walmart for $80 USD.

The pilot could have actually been breaking some rules, without realizing it, if you are located within 3 miles of LNS (Lancaster airport).

At the time I was further than 3 miles away from LNS.

@CGCampbell: I've recently seen similar on an airport, apparently recording an air show. It was probably arranged with the organizers though.


text:
Is line training (pay to fly) allowed in the USA?
In Europe some commercial airlines--such as Wizz Air and Air Berlin--charge licensed co-pilots to sit in the cockpit.
It is called "pay to fly" or "Line training".
Is it allowed in the USA ? Is there a FAA regulation regarding that?

I think GoJet did this a few years back but I don't have enough firsthand knowledge to post it as an answer.

Related: http://aviation.stackexchange.com/questions/1736

It is not prohibited in the US, but I don't think it's currently practiced anywhere.  There used to be a school called Tab Express that did something like this, but the most notorious one was an airline called Gulfstream International Airlines that operated as United Express in Florida.  Gulfstream is now operating as Silver Airways and, as far as I know, has discontinued the practice of having the FO pay to be there.


text:
Why are aircraft tires not pre-spun prior to landing to preserve them?
Why has someone not designed a landing wheel with a fin or fins on it so that the air will start the wheels turning before the wheels touch the ground? Wouldn't that preserve the tires longer from wear? Or would it make the control of the aircraft more dangerous in some circumstances, such as rain or snow, to have the wheels already turning when landing? If so, perhaps the fins could be manually or computer controlled for various weather conditions.

You'd need to match speed exactly which is very hard. plus there is the weight issue, anything superfluous that is heavy will not get on board

@ratchetfreak You wouldn't need to match it exactly at all! You'd just need to get closer to the actual ground speed than a stationary wheel is. Don't fall into the trap of failing to do anything just because a perfect solution isn't possible. (But I agree that the reason it's not been done will be that the advantages don't outweigh the costs.)

How do you spin the wheels in a way that does not affect their friction upon contact? I mean, if you use any kind of clutched drive, how do you ensure the clutch does not still contact the axle upon landing; any contact of the clutch would cause it to potentially burn out. If you use fins and airflow, would there possibly be enough airflow to overcome the inertial weight of the total tire/hub/etc.

This guy claims to have designed some hubcaps with fins in then that catch the wind and supposedly spin the wheels up after the landing gear is deployed

possible duplicate of How long do airliner tires last? Can this be improved?

I asked this exact question back in the old days when I was in the Civil Air Patrol. We were at an encampment at Chanute AFB in Illinois, and being given a tour of a B-58 they had on hand, whose tires were very badly worn. The instructor was telling us that the aircraft required new tires after about four landings. His response to my question was two parts: 1) anything that adds weight or complexity to a simple system is a bad thing, and 2) defense contractors don't make money by not selling you new tires. ^_^

@MattHill   IIRC  Frank Capra (the movie director) claimed that he was involved in development of  a similar hubcap back in the 1930s.

This has been researched over the years, but it seems that the solutions did not find wide application. The oldest I could find is from 1941 (see page 112 in the September 1941 issue of Popular Science), and there have been several attempts to implement a spin-up turbine. See “Wheels with wings” on NewScientist Blogs or “Spin Wheels Before Landing”, a discussion thread on Eng-Tips forums, for more. The biggest problem seems to be the inertia of the spinning wheel because it makes the aircraft react in funny ways if the pilot wants to correct his approach.
The rough field package on the Cessna Citation 500 uses this technique (for the nose wheel only) to protect the fuselage from debris which could be kicked up by the wheel spinup.

Control issues make sense! Gyroscopic precession is weird.

So if gyroscopic precession is an issue, does it have the same effect during take off?

@MKYung that's why people tap the brakes on takeoff

Also, after take-off you mind less when the aircraft rolls a little when you yaw and vice versa. This is quite different to trying to line up with the runway during an approach in gusty air.

And that's why Concorde had a disk brake on the nose wheel that was activated during gear retraction.

In the late 80s three German schoolboys invented “Aircraft tyres having an autorotation device” (rotation through airflow). They participated in “Schüler experimentieren” (which is “Jugend forscht” for kids under 15y). A picture is shown here (Engl. translation). AFAIK a major tyre manufacturer supported them but let it eventually disappear into a drawer.

broken links in answer...

I've also read in once case that spinning up the wheels before landing significantly increased the stopping distance.  Seems a little far-fetched but I definitely recall reading that.

@quietflyer I read that too.

It's just not cost-efficient. Any weight-add produces more fuel consumption, any systems produces more maintenance costs. It's just easier to change the tires if they're worn out.
source: Aerospace engineering

could you expand your answer a bit? the content is fine, but expanding the reasoning would help.

I thing Hubschr is correct.  In general the more things you add or make complex, the more things break and drive up the cost of maintenance.  Also when a tire goes the mechanic knows how to change it.  When some fancy electric motor or its controller, or it's power supply , or ...  it's a much bigger deal to troubleshoot and fix.  This can result in more down time.  Planes only make money when they are working, so keep it simple and get back in the air making money.

Well, this is a generalization and in this case I do not agree. ^^
Progress is most of the time not the effect of simplification. Systems are getting more complicated.
(As an example: If you would build a special rim (when you have retractions wheels) which use the airflow for generating the spin it does not have to make the maintenance costs higher and it maybe also not produce more weight. And (maybe) the over-all cost is better, because fewer tires worn out.) Source: My two cents as an engineer

A design based on the design of simple 3 cup anemometer would be lightweight, easy to understand and easy to maintain.

A good summary of factors in this concept is SAE AIR5800 report. It lists the pros and cons as follows, citing several SAE and NASA documents, and Boeing NACA and AIAA papers:
Advantages:

Reduces tire wear
Potential to reduce touchdown loads
Reduces debris spray on gravel runways

Disadvantages:

Does not reduce tire wear significantly for most airplanes
Not a reliable means to reduce landing loads, (and may even increase loads)
If employed, motors add weight, cost, complexity
Reliability expected to be an issue
Failure mode requirements minimize potential loads benefits
Additional maintenance
If employed, special tires require more testing and spares (left and right tires)
Causes noise and vibration at landing approach
Adversely affects on-ground indication systems using wheel spin-up signals upon landing

A biref snippet from the summary section states:

There may be solutions to many of the issues associated wirh tire prerotation, but in the end, the main reason tireprerotation is not practical on most aircraft is that it is simply not effective in reducing either tire wear or landing loads, andwould create more problems than it would solve.
There may be applications where this conclusion does not apply, such as for airplanes operating on gravel runways, or for airplanes that land at extremely high speeds incurring a higher percentage of tire wear at landing.

Perhaps the most notable case of considering pre landing spin up was for the space shuttle, but even with wear reduction of up to 50%, the idea was scrapped.
the SAE report can be googled, I will not provide a link to it as it is made commercially availlable by SAE, and I do not wish to violate their copyright


text:
Do I need a digital or analog scanner to listen to Air Traffic Control?
I'd like to buy a scanner to listen to Air Traffic Control (e.g. Tower, Approach, etc.). It would be used mostly at either Washington National [DCA], Dulles [IAD], or Baltimore [BWI]. Would I need a digital or analog scanner to hear these transmissions? Thanks!

You want to buy the scanner or you want to hear the ATC? Those are different questions. Try LiveATC. Not sure it coveR area you want or not,

LiveATC has fine coverage of the DC area, although it tends to be delayed by about a minute.

The voice communications for air traffic control is all analog (AM to be specific). A scanner that works in that frequency range on AM should be fine. Although a switch to digital has been considered, there is just too much coordination and investment required.
A good reference to find the right frequencies is AirNav. It will list all the frequencies for an airport.
Another good reference for many frequencies, not just airports, is RadioReference. The database there will tell you exactly how each channel is transmitted, so you know what type of radio you will need to listen to it.
Also keep in mind that ATC transmissions are line-of-sight, so you will have to be close to a ground station to hear it. The "rubber duck" antenna included with a lot of scanners is fine but a better antenna will help to improve reception. Also, military aircraft use a higher frequency range, and a lot of cheaper scanners will not receive this.


text:
Can a Boeing 777 be hacked?
All this confusion over flight 370 has me wondering if it would be possible to hack into an airplane and control the autopilot or other on-board systems in a way that the pilots couldn't regain control.
I know some basic searching reveals some articles such as this one. If the plane can't be hacked, could someone with the proper knowledge snip or disable the pilot's override?
Disclaimer: I know 0 about aviation, but I am a programmer. So my guess would be that anything that doesn't have a physical disconnect would be vulnerable to being hacked without being able to override it.
Additional Disclaimer: This is a question about curiousity / possibility, not a how-to.

most systems on an aircraft do have a physical disconnect, and the fly by wire lets the pilot full control if he needs it

@ratchet physical disconnect? How can I envision that -- like Scotty crawling through smoking engine shafts?-- My favourite joke is from the German c't magazine... an airplane passenger opens his notebook and freaks when he sees this dialog pop up: "New Bluetooth device detected: Airbus A380. Reset [yes] [no]?"

@PeterSchneider I meant there is a fusebox right behind the pilot where he can turn off the power to just about all systems

@ratchet ok, but he wouldn't like that during takeoff or landing. Or flying, as such.

How about stowing a trained monkey in the wheel well. It may not count as hacking, but you might be able to accomplish the same objective. A trained monkey could be really bad news, especially if it was charming, had a snazzy outfit and had evil intentions.

@PeterSchneider "He wouldn't like that". The point is that any electrical system might malfunction during flight and might malfunction in a way that harms the plane, either by causing problems for other equipment or perhaps even just by bugging the pilot with spurious alarms. Essential equipment on aircraft has redundancy built in; inessential equipment is, well, inessential. So, in some circumstances, the safest thing to do is to turn off some piece of equipment and the crew must be able to do that. Sure, they don't like it but they like the alternatives less.

Nothing can't be hacked. Therefore, yes.

@geoff Sure, if you live in Maximum Overdrive.

As of currently, the answer to this question is in principle no for commercial aircraft, at least not remotely. There are two parts to this:
From a system perspective:

Aircraft systems could probably be 'hacked'- assuming you could for instance screw up the flight computer by changing the chips in the belly- but there is no way you could really pull this off without the pilots noticing the course being way off from the compass for instance, so they'd figure out and shut it off. Many systems are not connected in a way that allows them to be cause damage to others. Systems are also pretty dedicated- if it's supposed to fly the plane, that's what it does, if it's supposed to navigate, that's what it does.
Systems on aircraft generally a have limited, if any, connection to the outside world. There are a few systems that have the ability to connect to the ground, and this is generally limited to simple text messages. Most of the electronics on the 777 are from the type's introduction in 1994. A decent analogy I think is trying to hack an early cell phone remotely.

And from a practical standpoint:

Aircraft are generally very stiff when it comes to changes- they are often approved in a certain configuration, and that then sticks, so there's little need to change stuff, and hence it's not possible. This applies to the autopilot and flight controls- once it's there, it stays there.
As a few people have pointed out, these systems are proprietary and closed source, making it difficult to program them if you're an outsider, but in theory it might be possible.

The fact that no aircraft, even the computer-intensive Airbus, have really been hacked to any extent, the notion that the entire aircraft would be taken over and flown somewhere is impossible.
As for original question link:

“The FAA has determined that the hacking technique described during a
  recent computer security conference does not pose a flight safety
  concern because it does not work on certified flight hardware,” he
  said.
“The described technique cannot engage or control the aircraft’s
  autopilot system using the FMS or prevent a pilot from overriding the
  autopilot,” said Dorr.
“Therefore, a hacker cannot obtain ‘full control of an aircraft,’ as
  the technology consultant has claimed,” said Dorr.

On a similar accord, I think I heard a story about a Bombardier Dash 8 that got a computer virus once, but I think it never got beyond the navigation system, the most processing intensive part. Unfortunately, I can't find a source for this.

Also, from a software development perspective, hacking a set of avionics would involve massive amounts of work and specialized knowledge.  Home computers and business computers (servers, etc) are run on common operating systems, like Linux, Windows, Unix, OSX, etc.  Avionics, not so much, they're proprietary and getting ahold of the schematics would be difficult if not illegal.  You'd have to to have incredibly specialized system knowledge to hack that system in the first place and that's even before you run into all the problems that you've mentioned.  Point being, hacking is highly unlikely.

Since I worked in the automotive industry: Any modern vehicle is fully wired, if only for diagnostics. If you get at the diagnostics port you can most likely hack the vehicle. @JayCarr: Yes, you would need immense specialized knowledge. It was frequently difficult to e.g. flash car devices although we were legit and had the encryption keys, hardware and documentation. But then it's sometimes difficult to log into a windows computer -- doesn't mean it can't be hacked. If you drill at the right location you may get to the wire, in case the port is outside or in the pilot's footspace ;-).

@PeterSchneider More what I meant is that you're more likely to see a Windows hacker because there are, literally, billions of users and billions of boxes to practice on.  Avionics software...not so much.  But yes, still hackable if you had a mind to do it ;).

+1. Also, people seem to frequently have a "movie" view of hacking, where any piece of copper or antenna can be tapped into and magically used to control the device. This isn't really true; imagine, for example, trying to hack a computer through a microphone jack or a VGA port, or trying to flash a car ECU through the radio's FM antenna, or trying to steal pictures off a cell phone through the GPS transceiver (well, presuming reasonable vendor extensions on that last one).

I agree with @JayCarr the systems are highly proprietary so it would take a really good inside knowledge and then like this answer says it is probably near impossible to do it remotely. And if you're going to physically take apart systems why screw with the navigation why not just put a bomb or something? I mean I get that piracy is one of the theories but there was a Lear Jet that crashed 3 miles from an airport in New Hampshire (USA) back in 1996 it took 3 YEARS to find it. Oceans are much bigger.

Regarding the security researcher: i saw his conference presentation. It was clearly hookum. "I am not an expert and don't have access to a real plane, but look what i can do to a flight sim on my computer! That means i must be able to hack a plane! Planes use radar, so i must be able to do something eViL with radar to hack a plane!' and other such crap

@Jason C: But vanilla phones have been hacked through Bluetooth; and isolated PCs have apparently been hacked by the NSA via radio. The latter seems to have involved planted radio hardware though, i.e. access to the device at some point. But to plant (cleaning? loading?) personnel at some (remote?) airport who can install a radio device seems not inconceivable.

@peter Sure phones have been hacked through bluetooth but that wasn't my analogy...

@Jason C: Then let's use... GPS? It's cryptografically signed so that only the key owner could spoof ... wait, that would be the DoD ... ok, the DoD injects a malicious GPS signal into the plane with Snowden on it, buffer overflow, injecting code. GPS and thus autopilot think the plane is somewhere else. Pilot is napping over the Pacific. Does that count?-- Even a plain reboot can be exploited: In one of our cars the user crashed the head unit purposefully by zooming violently in and out of the navigation map. Head unit reboots while driving -> user can watch DVD on the front monitor :-).

@PeterSchneider My analogy was stealing pictures via GPS, so no, your example is unrelated and doesn't count. I am having trouble seeing your point? If you are trying to make a point you could at least be consistent and not forget to mention that computers and car ECU's have also been hacked through means other than the ones in my analogy. Be more clear and less confusing and state what you mean. Your example cases are all 110% real but your direction is not apparent. You did understand my analogy, correct? Sorry if it was unclear. Remove the cell phone example from it if you wish.

http://www.wired.com/2015/05/feds-say-banned-researcher-commandeered-plane/ I just saw this in my news feed. Does this change anything in regards to your answer?

@Tom the details still seem to be very vague and some industry folks aren't buying the story without more details. I'm going to let the storm settle first before I get around to rewriting as appropriate :)

@JasonC "trying to hack a computer through a microphone jack or a VGA port, or trying to flash a car ECU through the radio's FM antenna," Is a hack trough the VGA port, or through an FM antenna, inherently absurd and impossible?

There are certain communication systems that theoretically could be hacked into.  CPDLC is used to communicate digitally between pilots and controllers, so if it were to be hacked someone could send instructions to the pilot that would look like they were coming from a controller, and the pilots would not know the difference.
Flight plans can also be uploaded to the Flight Management System in some cases and could be used to program a different route.
One thing to keep in mind though is that the pilots are still intimately involved.  They see and approve all changes.  Minor changes might go unnoticed, but if they were instructed to fly in a completely different direction or to a different destination, they would probably start asking questions.  If they were sent directly towards a mountain or  another airplane, they would likely notice or other safety systems would step in to warn them.
Even in a "worst case" scenario where someone were able to reprogram the autopilot (I don't believe that this is even possible right now) then the pilot can always just turn it off and fly manually.
In short, this isn't really something to worry about.

Everything is possible. For example, NSA could have forced a manufacturer to include some backdoor in the system for their use.

Haha, I like the way they come up everywhere ;)

As somebody who actually wrote automated test software for some controller going into an airplane and looked into the procedures which are done to ensure  that the part  goes into the airplane in a 100% documented and known state, I think that this is highly unlikely. If anything they would have to hack the manufacturer and add additional (which I think is easier, but very criminal, since you would be breaking the laws of dozens of countries at the same time).

@Sascha Yet that A400M crashed because of bad software (we are told).

"Hack" has a variety of definitions, from reading private data off a non-critical system, to disrupting or confusing an avionics mode, to taking control of the plane and locking out pilots.  This complicates the discussion of how likely planes are to be "hacked".
In general, the recent consensus seems to be that under some definitions of "hack", this certainly is possible, even without insider knowledge. There's some reports of successful hacks out there, though since the details are confidential it's hard to tell what researchers accomplished or their method of attack.   Both researchers and plane manufacturers seem to agree that taking control of an airplane for more than a few seconds would be extremely difficult, but not impossible,  and that's even with insider knowledge or prolonged access the plane.
Current policy guidance by the FAA can be found in PS-AIR-21.16-02.  I've heard that the FAA will eventually recognize DO-326 as a means of compliance.
I'm going to provide some information on safety and integrity measures that are standard in the industry and make hacking avionics difficult.  In particular, hacking avionics is a type of safety-critical embedded device hacking, not server or personal computer hacking.  Safety critical embedded devices have a much smaller attack surface and many more mitigation methods in place, as outlined below.
Threat model and differences from computers
The threats you face on planes are very different from servers and laptops. Especially for the most critical systems like displays and autopilots, datatypes are strict, messaging is scheduled, processing time is limited, and strings are rarely used. To accomplish most traditional attacks, you would have to break these protocols. This is similar to other kinds of embedded programming where the software is so inflexible that the attack surface is small.
Some things that definitely help attackers here include physical access to the software update process, being able to work with the plane for several days, insider knowledge, simple RF equipment, or ability to hijack satellite communications to the plane.  If your attacker has some of those the threat is much more serious.
Availability
Is it possible to mess with the availability of aircraft systems?  Yes, if you can manipulate the right signals.  You could, for example, make the flight control computer think a sensor has gone bad.  You could throw off a lot of sensors with powerful enough radio signals.  A bug in 787 NG display software once led to all displays going black. Also, most flight computers use off-the-shelf processors with lots of weird behaviors, and especially if an exploit lets you execute arbitrary assembly code, you could exploit some of these to take the avionics system offline.
However, availability isn't as big of a concern as integrity of the avionics.  Most people are really concerned about a hacker taking complete control of the plane, not forcing the pilot to fly by hand.
Partitioning
Different functions, especially ones at different safety levels, are by regulation insulated from each other.  Although aircraft have historically had dedicated hardware for each component, nowadays much of this software is run on shared LRU computers (this is called an IMA architecture).  But despite sharing hardware, the software processes are strictly insulated from each other and it is proved during certification that no processor faults, error conditions, overflows, etc. could transfer from one process to another, especially not a higher level process.  Yes, signals pass between partitions, but any signals passing from a lower safety level to a higher level are individually justified to ensure they won't cause safety issues.
In order for an exploit to have catastrophic effect, it would have to get around these by either by 1) working directly with level-A hardware and processes 2) finding a poor assumption about the impact of a lower-level signal.  These poor assumptions exist, but they're rare.
See DO-297 and DO-178C for more information on this partitioning.
Testing
Many hacks in personal computers happen because of inadequate testing that lets exceptions and faults into the system.  Level A software is tested far more comprehensively than most apps or PC software. Every line is tested for MC-DC coverage (not exhaustive, but every decision has to be exercised as both True and False). Structural coverage is also evaluated to make sure unintended interactions don't occur.  If faults do occur, the RTOS is designed to take care of these faults predictably and reliably.
Software updates
Ok, say you can't find a way to execute arbitrary code, can you mess up the software update process?  This initially seems viable, especially given the new tendency to update avionics firmware over networks.  There are several issues here.

Most aircraft are not directly connected to the internet for avionics updates.  So you'd have to get access to the local network for updates and then also get access to the planes using this network.
Most software updates use a high-fidelity integrity check, for which you'd need an incredibly difficult hash collision. Some update processes don't have high fidelity checks, but if this is the case they must qualify to authorities that their update process is stable and error-proof.  There are flaws that could be exploited, as the issue found during the A400M Seville crash showed.  So only option would be to forge the CRC as well for the update and/or to find an exploit in a qualified loading process.
Network based updates also keep track of the version numbers and may keep track of the integrity checks for the software.  So you can't exactly force an update from version 13 to a hacked v. 13.1 without someone noticing.
Currently supply chain attacks are becoming more and more frequent.  While I'm sure a supply chain attack might be feasible, regulations such as DO-330 tool qualification and DO-178 configuration control have protected against inadvertent changes during software builds for years already.   Tested and reviewed versions of code are under careful control and compared against the final versions, software build environments are locked down and qualified, and lists of included libraries are documented and approved.  You'd have to find a blind spot in these processes to attack the software supply chain.
Software updates (not navigational database updates) are performed infrequently, maybe once every three years if that, greatly mitigating the opportunity to hack a software load.  Interlocks are in place to prevent reloading the software when not desired. This infrequency also leads to greater scrutiny of the entire process.

For more analysis of safety measures to prevent corrupt software or databases from being uploaded, I'd suggest you consult the standards for this process: DO-200A, Chapter 5 of FAA Order 8110.49,and FAA AC 20-153.
Manual and Automatic Disconnect
Usually the autopilot and other software is managed by another system which has interlocks and disengage logic. These have safety standards and tests written to make sure they work reliably, including a requirement for a software-free method of disengaging autopilot.  This make it very tricky to find an exploit here without, say, already being able to execute arbitrary code.
Even if this disengagement software doesn't work, pilots have circuit breakers and can disable the entire avionics system, then fly the airplane by hand.  See BigHomie's answer for a discussion of how feasible it is to hack this (answer: not very).
In theory, this should work, but in certain situations the plane could be put in an unsafe situation before the pilot could regain control.  As the 737 Max incidents showed, sudden pitch down when close to the ground can be difficult to recover from, even if you can manually disengage the autopilot or manually disengage the trim system.
Note: I'm not an expert in avionics or embedded device security.  Hacking tends to be about thinking outside the box, so let me know if anything I assume here is inaccurate or if I miss anything.

"Software updates use a high-fidelity integrity check" We were told that the A400M crashed in Seville because the fuel control software was corrupted...

@curiousguy In that crash the torque parameters were wiped during a software load, although I can't find out what that means exactly. It sounds like they weren't following directives to make sure their "update process is stable and error-proof.", and even though the software program itself was configured correctly (matching CRC's and such), some of the airplane-specific data was missing.

Most aircraft don't get their updates from a network at all, they require a hard link to the storage medium holding the data/updates. Think a USB stick in modern aircraft or some other data cartridge in older ones (compact flash cards come to mind, or even magnetic tape). Of course those can be fed with corrupted data, but it adds yet another layer of security.

Any computer can be hacked. I'm not saying I have the knowledge to do it, or even know much about airplanes to tell you how. However if someone with the knowledge required had the proper time, motivation, and access to an aircraft, it could most definitely be done. If you're asking if a plane can be taken over from the ground midflight, without any prior modification, I can't answer that because I don't know enough about the system.
Theoretically discussing how to hack a plane midflight, with or without prior modifications, would probably get me flagged and placed on a list somewhere, so I decline to go into detail on that.
However, one must think about what wireless protocols are used for a plane to talk to the ground, what computer subsystems can access that physical link, and if those subsystems can control the plane, or access other subsystems that can control the plane.
'Locking a pilot out' probably involves disabling a manual switch somewhere, tricky to do via computer, but can be done given enough amperage was passed through the switch to blow it, or if it were rigged before hand (prior modification).
If we're talking how to, this is a episode of Numb3rs with a similar theme, however it wouldn't be far off from what would need to take place in real life.

Taking it over from the ground without prior access to the craft would be, for all practical purposes, impossible. Even if you did get access to the craft and compromised the autopilot, the pilot could just turn it off. Even if you managed to blow the switch, he could pull the fuse or switch off the breaker. And, barring that, he could just shut down the power bus it was using. The odds of someone successfully taking control of a 777 autopilot without having previously been an engineer involved in designing that system are extremely low.

Passing amperage through a switch to blow it would only work in an incredibly bad novel/film. It's not realistic in an avionics perspective. -source: Avionics technician for ten years...

@davewasthere why? I'm not doubting you, just curious, because from an Engineering perspective, the switch has a current limit, which hopefully is at least twice its nominal rating. Can you discuss what safeguards are in place to prevent damage?

Any computer can be hacked.  If the computer has no connectivity, only if you can get physical access to it. There are commercial systems, inside very secure buildings, with no connectivity to the outside world.  They are unhackable. The critical systems on aircraft have no connectivity except within the aircraft.  Therefore, you would need physical access to the aircraft.  Now we are in the realms of James Bond.

@Simon it can still be hacked, you just need physical access to hack it. By definition, aircrafts need remote communication

@BigHomie Er, I think that's exactly what I said ;)

Remote communication - so you'd need to hack the GPS signal from a dozen satellites, or take over the Gnd position location equipment (VORs, or the ILS at an airport). or takeover the comm's from ATC for several locations - Center, Approach, Tower, to convince the pilot to fly off course or something.

@CrossRoads: To send a plane off course by playing with an airport's ILS, you'd have to physically move at least the localiser antenna (and possibly move the glideslope antenna as well, or at least disable it). The ILS doesn't carry any encoded information; you just follow the beam down to the runway.


text:
Does training adolescents require a different approach with regard to the level of neurological development?
There is some evidence of brain under development in adolescents, those from 13 through as high as 23 or 24, that is linked to judgment, decision making, risk appetite, etc., mostly in an unfavourable way as it relates to properly evaluating a flight.
See:  http://www.nimh.nih.gov/health/publications/the-teen-brain-still-under-construction/index.shtml
Does this suggest that CFIs training an adolescent should approach the training differently than how they train adults?  Are additional training and scrutiny indicated in order to minimize the effects of an under developed brain in this population group?  Or does doing so somehow cause some level of unwarranted discriminating practices to an adolescent training for his pilot certificates?

As an instructor at a fairly large university, I see lots of students in the age group you mention. Most of the time, no adjustment to the training course is necessary. There will always be outliers, of course, but in general university students in the 18-23 age group have no problems keeping up with the training curriculum.
Anecdotally, the students that I've personally had the most trouble with aren't the ones that are younger, they're the ones that think they already know what they need to know. Which is just another way of saying that almost anyone with an sincere desire to learn can be taught to become a safe aviator.

By trouble, do you mean their stick and rudder abilities or decision-making, risk management cognitive abilities??

@david - cognitive. Stick and rudder skills are generally the easy bit.


text:
Is an Airbus A319 allowed to fly with a missing flap track fairing?
During a recent flight from Copenhagen to Milan in an Airbus A319, one of the flap track fairings were missing from the wing. I saw this during takeoff, and notified a flight attendent as soon as the "seatbelts on" sign was turned off.

She seemed slightly surprised and went to the cockpit (supposedly to tell the pilot). After some minutes she returned and told me that it was fine, the captain had seen it during inspection and it was noted in the log
Is it normal procedure to take off with knowledge of missing flap track fairings? What are the consequences of one missing?

Think of it like the plastic air deflector under the front bumper of your car -- the primary effect of losing it is a ding on gas mileage.  (The climb performance penalties are something the pilots would take into account in their flight planning, but would only have an effect on a situation that was at or near the limits to begin with.)

what the heck is a "flap track", by the way?!  (I mean I can see what it is - but I've always wondered what they are!)

@Fattie Feel free to ask a question about that. Ideally, without the profanities.

@Fattie those are usually long lead screws and are part of the actuator that moves the flaps. They extend outside the envelope of the wing itself and are usually covered with a cowling like that to reduce drag and keep the elements out of the mechanism.

ahh !  so it's like a big long Kohlmorgen (or whatever) actuator, and it pushes the flaps up and down!  of course (I guess that's why they are .. near the flaps .. and have ..  the word "flaps" in the name  :O )  duh on me.  thanks!  :)

If the airline wasn't permitted to fly a plane in this condition, did so anyway, and something went wrong, the liabilities would be crippling for the company. On those grounds alone, we can be pretty sure that it's OK to fly a plane with the missing fairing.

Fully permitted according to this Configuration Deviation List for the A320:

One fairing may be partly or completely missing.

There are more posts about this occurring. As Noah Krasser points out, it looks fairly dodgy to observant passengers. This is the "Master" list, i.e. the safe and approved baseline. Some airlines may have changes in their own internal documents not to dispatch aircraft in this condition at all.

π % fuel increase .... that's a lot !

It may look dodgy but it's not like it's a safety-critical component

@Antzi yea a lot, especially for easyjet!

THREE PERCENT!  no way.  also, it's not clear if they mean the front or rear fairing; slackers.

@Fattie, you are correct it's not 3%, it's 3.14%. Also the note clearly states what do regarding the front or rear fairings.

thanks, does the 3.14% refer to the front one missing {ie, in fact both must be off} or does it refer to the rear only one missing ... does anyone know?

The flap track fairings are not obligatory for safe operation of an aircraft.
I found a post on Airliners.net, regarding a B757:

Straight out of the MEL/CDL manual for the 757, The item falls under CDL 27-51-1 which states there are 6 installed [Flap Track Fairings, Remark] on the aircraft and up to one per side may be missing. No fuel burn penality but there are a few small weight penalities, no speed penality. Our A-300's with the same CDL item does have a 1% fuel burn penality and only one can be missing out of the 10 installed. I see this item come up 3 or 4 times a year, no biggie.

However, some Newspapers overact such things a little. It's about the loss of a flap track fairing on a 737 by the dailymail.

Pilot tells 109 passengers on plane that part of the wing had 'fallen off' and is then forced to make emergency landing

[...]

Lorenzo Hernadez Jnr took to Facebook to speak of what he described as his 'near death experience.'

[...]

'I heard a few people to the side of me scream to the aircrew, a piece broke and flew off and a panel was hanging.'

[...]

He goes on to describe how people were 'praying' as the plane experienced 'the worst turbulence ever.'

Source

Looks like a standard Dailymail article.

@VladimirF Ah, good 'ole Daily Fail.

Come to think of it, if the 109 passengers were all 20mm in diameter, this could've been true...

authors: Vladimir F Героям слава


text:
Does adding more engines to commercial aircraft increase safety?
For example, the Boeing 747 has four engines, whereas the Boeing 737 has 2. Does this means that the Boeing 747 would be safer if one of the engines fail?

No.  the 737 is perfectly safe on one engine.  It needs to demonstrate this during certification.

i recommend reading up on ETOPS.

The number of accidents due to engine failure on transport aircraft is so small that any statistical analysis would have low confidence

I narrowed down the question to commercial aircraft since it was broad and I think that is what you were interested in. Single vs. Twin engines on GA aircraft is a whole different discussion and question IMHO....

Not really, I wanted to ask why some planes like the B-52 contain 8 engines, while the mammoth A380 is okay with 4 as well.. So, you can have enough power with fewer engines as well, then why are more engines added?

B-52 was built in early age of jets where 8 engines were needed. there was a proposal to re-equip the plane with 4 engines but turned down due to TCO. Compare thrust of B-52 at 17,000 lbs each vs 70,000 lbs each on A380. Imagine 1 engine of 380 equal FIVE of those on B-52!! It is more about technology in the end.

@AbhinavAgarwal You could (thrust wise) replace the four CF6 engines on the Boeing 747-400 engines with two GE90-115Bs as used on the Boeing 777-300ER. Anything above four engines I believe is more a technology limitation and thrust requirement compromise than a safety requirement.
If I screwed up the question feel free to reformat the question back or change it as you want :)

In the modern day and age, it is very difficult to put a numerical value to this type of question, simply because the chance of something happening is so incredibly remote. This is not a clear-cut equation and is dependant on several factors.
A few thoughts:
The impact of loosing one engine will be less significant on a four-engined aircraft than a two-engined aircraft. You only loose 25% of thrust as compared to 50%. However, two engined jets are built with greater safety margin and are able to handle an engine failure at any phase in the flight. Four engined jets are certified to be able to fly on two engines on one side of the wing.
In theory, you are twice as likely to suffer an engine failure with four engines, since you have twice as many engines. While the lost thrust would be less significant, very rarely damage can propagate to cause damage to other parts (See QF32 and ELY1862). In general, it is easier, cheaper and faster to maintain two engines than four. Should a complete engine swap be necessary for instance, you only need to do it twice instead of four times. Every extra step is an step where something could theoretically could go wrong.
The GE90 (Boeing 777) is certified to handle 'large' birds (4kg) as compared to the CFM56 (Airbus A340) only being able to handle 'medium' birds. Hence, the Boeing 777 engines are likely to be a bit more robust.
From a human perspective, pilot workload will be lower with two engines rather than four. Should something happen in a time-critical emergency, there will be more systems you need to keep an overview of. See CAL006.
Furthermore, not every engine-related incident is spared simply cause you have four engines. BA Flight 9 lost all four engines due to volcanic ash. On BA Flight 38, the engines failed in unison due to ice in the heat exchanger on the engines that were operating in identical conditions.
Despite ETOPS being around for several decades and applying to thousands of aircraft I have not heard about a single accident involving commercial jets operating in that domain.

That is what we call a perfect answer!

'able to handle an engine failure at any phase in the flight'.. what about duel engine failure at 1000ft after takeoff?

The more engines you have, the better off you are if one engine fails; however, the more likely it is that one will fail.
Engines, like many things, are rated in terms of "mean time between failures" or MTBF. For aircraft engines, MTBF is relatively high (tens of thousands of hours when properly maintained), but they do still fail in relatively unassuming situations.
It's simple probability that, given a nonzero chance for a given engine to fail, the chance that at least one engine will fail is the combined chance for each particular engine to fail. So if the engine type used on a 747 has a 1/1000 chance to fail on a particular flight (the actual chance is much lower, in the tens of millionths), there is a 4/1000 = 1/250 chance of at least one engine on the plane failing during that flight. If the two engines on a 737 each had the same chance for failure, the chance of one engine failing on the flight are half those of one engine failing on the 747, because there are half the number of engines that could fail. In MTBF terms, we can say the mean time between single engine failures on a 747 is half that of a 737.
So, we see that increasing the number of engines, all other things being equal, actually increases the chances of something going wrong. However, the pilot of a twin-engined craft, though he may be twice as likely to experience an engine failure compared to the pilot of a single-engine craft, is going to have a much better day when it happens than the single engine pilot will.
The big question that really has to be asked is, how many engines can the plane fly on, and how many more than that number does it have? Jets like the DC-10 and 727 were marketed for their relatively high fault-tolerance; they could maintain level flight at low altitude on only one engine, having lost the other two, and on the 727 it didn't really matter which one was still running as they were all so close to the centerline. The 747 requires at least two of its four engines to maintain level flight (see British Airways Flight 9 in 1982, during which all four engines quit mid-flight due to ingestion of volcanic ash; restarting one engine slowed the rate of descent but it took two to level off).
Now, primarily for fuel efficiency reasons, passenger jets are designed with as few engines as they need; engines are most efficient at a specific point in their power curve, and you design the aircraft to cruise efficiently at that power output of its engines. This is why the 737 (and newer jets like the 777 and 787) doesn't have four engines; it doesn't need them, because the 737 is a fifth the weight and the engines it does have are more efficient than the 747's.

From a strictly "engine cut out" point of view in todays day and age you are not getting your self very much. There is a great article here that talks quite a bit about this point from a GA stand point. There is of course the simple fact that for some planes you need more thrust to simply fly. There has also been lots of research done on if servicing an engine frequently makes it safer or actually adds to the problem. Again from a GA (mainly piston) standpoint there is a case to be made.

I edited the question to narrow it down to what I thought was the OP's interests. Perhaps you would consider making a new question specifically for GA aircraft and posting this answer there instead? :)

ICAO regulations allow two-engine aircrafts to fly over oceanic routes (they must have a Ram Air Turbine - RAT) so such a plane - certified as ETOPS, Extended-range Twin-engine Operational Performance Standard - must be safe. Anyway I can agree with other answers stating that if you have to loose an engine it is better to have 3 other.


text:
What is an Aircraft Overhaul?
While reading the FAA presentation on Part 135 Airworthiness Requirements, on slide 7 where it discusses maintenance, the list includes

inspection, overhaul, repair, preservation, and the replacement of parts, but excluding preventive maintenance

I am sure that the other regulatory bodies have their own, similar lists.
I know that an aircraft overhaul is one of the major processes involved in (especially commercial) aircraft maintenance. What exactly is meant by overhaul? What is done to an aircraft when it is overhauled? And finally, why is it so important for an aircraft?

Could you perhaps elaborate your question?

Done! Is it okay now? Anyways, the answer will make the question absolutely clear to you.

But if I don't even understand the question, why would I care for the answers?

Does this edit help the down-voters?

@CGCampbell it does, but the question still needs to be explicitly limited to commercial jets.

@MikeFoxtrot Why does this question need to be limited to commercial jets? Plenty of piston-engined and turboprop aircraft operate under part 135.

@Pondlife the thing with Part 135 was something CGCampbell added two edits ago and was not part of the original question. Part 135 does indeed cover many aircraft, I am just trying to scope down the question to stop it from what I see  is becoming very broad; Feel free to modify as you please.

14 CFR 43 MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION applies to all aircraft regardless of their use. That is where the answer is to be found.

Briefly, "overhaul" isn't a strictly defined term but it's generally used to refer to the intensive inspection, maintenance and repair of an aircraft component, frequently - but not exclusively - the engines.
The word overhaul by itself is a general term that's used in all sorts of places but it's not defined in 14 CFR 1.1, for example. The most specific definition I could find is in this agreement between the FAA and EASA:

"Overhaul" means a process that ensures the aeronautical article is
  in complete conformity  with the applicable service tolerances
  specified  in the type certificate  holder's, or equipment
  manufacturer's instructions for  continued airworthiness, or in  the
  data which is approved or  accepted by the Authority.
No person may
  describe an article as being overhauled unless it  has been at least
  disassembled, cleaned, inspected, repaired as necessary, reassembled
  and tested in accordance  with the above-specified data.

Note that that definition only mentions "aeronautical articles", which is very broad indeed and not limited to just engines, airframes or anything else.
In the general aviation world, a very common usage of "overhaul" is in the context of piston engines, which have a Time Between Overhaul (TBO or TBOH) set by the manufacturer. Once an engine reaches that number of hours, it needs to be overhauled. But even there, the term isn't precise and the FAA has a circular called Reciprocating Engine Overhaul Terminology and Standards that explains what it usually means:

c. Overhaul.  In the general aviation industry, the term engine
  overhaul has two  identifications that make a  distinction between the
  degrees of work on an engine:
(1) Major Overhaul.  A major overhaul
  consists of the complete disassembly of an  engine. The overhaul
  facility inspects the engine, repairs it as necessary, reassembles,
  tests, and  approves it for return to service within the  fits and
  limits specified by the manufacturer’s  overhaul data. This could be
  to new fits and limits or serviceable limits. The engine owner  should
  clearly understand what fits and limits should be used when  the
  engine is presented for  overhaul. The owner should also be aware of
  any replaced parts, regardless of condition, as a  result of a
  manufacturer’s overhaul data,  SB, or an Airworthiness Directive (AD).
(2) Top Overhaul.  Top overhaul consists of repair to  parts outside
  of the crankcase, and  can be accomplished without completely
  disassembling the entire engine. It can include the  removal of
  cylinders, inspection and repair to cylinders, inspection and repair
  to cylinder walls,  pistons, valve-operation mechanisms, valve guides,
  valve seats, and the replacement of piston  and piston rings. All
  manufacturers do not recommend a top overhaul. Some manufacturers
  indicate that a powerplant requiring work to this extent  should
  receive a complete overhaul.

Apart from that usage, the term MRO (Maintenance, Repair and Overhaul) seems to be used in the airline industry to describe maintenance facilities, but there isn't any specific definition that I could find.
So there's no single definition of "overhaul". It just means that you've very thoroughly inspected and tested something on an aircraft, whether it's an engine or a wheel, and you've verified that it's fit to be used in flight, i.e. it's airworthy. The reasons why it's important are probably obvious; they're the same reasons that your car needs regular servicing to run safely, efficiently and reliably.

Overhauled.
  Describes an airframe, aircraft engine , propeller, appliance, or component
  part using methods, techniques, and practices acceptable to the Administrator, which has undergone the following:
  (1)
  Has been disassembled, cleaned, inspected, repaired when necessary, and
  reassembled to the extent possible.
(2)
  Has been tested in accordance with approved standards and technical data, or
  current standards and technical data acceptable to the Administrator (i.e.,
   manufacturer’s data), which have been developed and documented by the holder of one of the following:

TC,
Supplemental Type Certificate (STC),or article approval under § 21.8, or
PMA.

Rebuilt.
  Describes an aircraft, airframe, aircraft engine, propeller, or appliance, using new or used parts that conform to new part tolerances and limits or to approved oversized or undersized dimensions that has undergone the following:
(1)
  Has been disassembled, cleaned, inspected, repaired as necessary, and
  to the extent possible.
(2)
  Has been tested to the same tolerances and limits as a new item.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2020-62E.pdf
Also peruse 14CFR43 PART 43—MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION
in particular 43.2

Records of overhaul and rebuilding.
(a) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being overhauled unless—
(1) Using methods, techniques, and practices acceptable to the Administrator, it has been disassembled, cleaned, inspected, repaired as necessary, and reassembled; and
(2) It has been tested in accordance with approved standards and technical data, or in accordance with current standards and technical data acceptable to the Administrator, which have been developed and documented by the holder of the type certificate, supplemental type certificate, or a material, part, process, or appliance approval under part 21 of this chapter.
(b) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being rebuilt unless it has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested to the same tolerances and limits as a new item, using either new parts or used parts that either conform to new part tolerances and limits or to approved oversized or undersized dimensions.

When dealing with regulations, you may not find an exact, all encompassing definition.


text:
What is the minimum separation between two landing A380s?
I have always assumed that landing separation depends on wake turbulence limits depending on weight class.
However I see that no minimum separation needed for the A380 as trailing aircraft (and in a few other cases):
http://www.ais.gov.hk/HK_AIP/aic/AIC22-12.pdf
In practice, what distance or time separations used in these cases, where wake turbulence isn't the limiting factor?

Related and discussion in airliners.net

In general, the heavier an aircraft is, the greater the wake it produces but, it is also less affected by the wake of other aircraft.  In particular, an aircraft can always resist the wake turbulence of a lighter category aircraft.
It was found that the A380 was not affected by wake turbulence from any aircraft in front, including another A380, down to the minimum RADAR separation used on approaches which is 3NM.
The RADAR separation used on any particular approach might be greater, but not less than this minimum therefore, there is no need to apply wake turbulence separation to a following A380 and the required RADAR separation for the approach is applied.

Thanks Simon I was looking for that 3NM value. And then I wonder, if the "particular approach" value is greater, is that found on the approach maps, or national ATC websites?

@DanielSparing Sorry, I don't know where to find the separation minima for a particular approach.  That's worth a new question I think.


text:
Why is the Inertial Reference System not used to determine true altitude?
Given three very accurate laser gyros and accelerometers I believe the Inertial Reference System (IRS) should be as accurate in determining true altitude as it is in determining lat/long, providing the aerodrome elevation was supplied at initialization.
I realize that pressure and density altitude are what is used in aviation, but could the IRS be used as a backup? Could it replace or backup a radar altimeter for approach/landing?

IRS will drift over time. Decision altitude is 200ft for ILS CAT I approach.  0.5nm drift is not to much for navigation but it will be huge difference in altitude and you can hit obstacles if your altitude has drifted just couple of feet. Probably it could be used if backed by GPS as EGPWS does or non baro RNAV approaches do. But even GPS precision alone is not enough in this case and you need WAAS.
Radio altimeter can give much better precision.

Another point: Just think what happens if you rely on inertial signals for altitude separation. By relying on atmospheric pressure, all planes in the area have a common reference. With an inertial altitude signal, the drift is different for each aircraft and would at least need an additional margin to keep aircraft reliably separate.

obviously, but that wasn't what i was asking for. I had my question edited (read: changed) twice to such a question though.